The black streaks
The dark streaks on many Florida roofs are caused by cyanobacteria, commonly Gloeocapsa magma, which feed on the limestone filler in asphalt shingles and spread with the wind and rain.
They start as small spots, then streak down the roof as water carries them.
Green film and mildew
Green algae, mildew and mold grow on stucco, siding, concrete and pavers wherever moisture lingers, especially on shaded, north-facing surfaces. They anchor into pores and textures, which is why a quick rinse rarely removes them for long.
Moisture and shade are the main drivers.
Why Florida
Warm temperatures, high humidity, frequent rain and mild winters give these organisms a long growing season. Newly built homes can show stains within a year or two.
Landscaping near the walls adds shade and moisture.
What helps
Treating the organism rather than only the color slows regrowth. Trimming trees for sun and airflow, directing sprinklers away from walls and using algae-resistant roofing products also help.
Zinc or copper strips along the ridge can slow roof algae.
